Serum albumin was found to be an important allergen in horse epithelia extracts. The albumin concentration varied considerably, depending on the raw material used for the preparation of the allergen extracts. In addition to serum albumin, two allergenic proteins were demonstrated in horse dandruff. The molecular weights of these allergens were calculated to be about 38,000 and 22,000, respectively. The pi of the latter was 4.2. Fifty sera from allergic patients were studied by means of the radioallergosorbent test (RAST) using horse epithelia extract, horse dandruff extract, a purified horse dandruff allergen and horse serum as test allergens. About 24% of the patient sera studied were found to contain IgE specific to horse serum proteins. IgE specific to horse dandruff proteins was found in about 62% of the patient sera, while the rest, about 14%, apparently contained IgE specific to both kinds of allergens.